1. Field of the Invention
The present invention relates to a steering wheel used to steer a vehicle, and more particularly, relates to a steering wheel whose ring plane constituted by a top plane of steering wheel can be aligned along generally vertical direction.
2. Description of Related Art
A conventional steering wheel includes a wheel body and an airbag apparatus for protection of a driver. The wheel body includes a ring for holding at steering, a boss disposed at the center of the ring and joined with a steering shaft and on which boss the airbag apparatus is disposed, and a spoke connecting the ring and the boss. The wheel body is coupled to the steering shaft in such a manner that a ring plane, i.e. a top plane, of the ring is disposed diagonally downward and rearward. JP2001-277983 A shows such a steering wheel wherein an airbag apparatus is activated upon an impact to inflate an airbag such that the airbag expands from inside of the ring toward outside, and eventually is formed greater than the outer diameter of the ring on top of the ring, thereby preventing a driver from hitting against the ring.
Another steering wheel system is shown in JP 2003-306122 A, wherein the ring supports an inflated airbag in such a stable manner that the airbag hardly moves out of place in the event of bumping against the driver. In this steering wheel system, whenever an airbag apparatus is activated, the ring plane or top plane of the ring is mandatorily aligned along the vertical direction. That is, a front part of the ring is raised upward. Accordingly, the front side of the inflated airbag is stably supported by the front part of the raised ring, which prevents a protection plane of the airbag facing toward the driver from moving forward at its upper edge and enables the airbag to receive the driver with sufficient cushioning property.
However, in the latter steering wheel system, the rotational displacement of the ring plane is conducted at the location of a tilt mechanism arranged on the steering shaft below the steering wheel. With this structure, the turning radius of the movement of the ring plane, that is, the radius from the tilt mechanism to the ring plane is large and so is the downward displacement of the ring before the ring plane is brought close to the upright orientation. Consequently, the moved ring is likely to narrow a space around knees of the driver.
JP 2001-277983 a mentioned above also describes a feature of a steering wheel wherein the entire ring plane aligns itself along the vertical direction by deformation so it is expanded in parallel to the front side of upper body of the driver when the steering wheel contacts with the driver upon an impact. Specifically, when the steering wheel contacts with the driver moving forward, the entire ring plane, which is usually diagonally oriented with the front part arranged upward and rear part downward, is aligned vertically by plastically deforming its predetermined portion, thereby absorbing an impact energy.
In the above steering wheel, however, the portion to be deformed upon the alignment of the ring has a uniform absorption property of the impact energy. The absorption property is not adjustable depending upon the size of a driver, the distance from steering wheel to the seated position of the driver, or the speed of vehicle.